1. Field of the Invention
The present invention relates to a heat exchanger for use with a combustion heater for effecting the heat exchange between air and hot combustion gases.
2. Prior Art
There are known various heat exchangers used with combustion heaters for effecting the heat exchange between air and hot combustion gases generated by burning gaseous or liquid fuels. Known heat exchangers generally include a tubular body, a number of interior fins disposed on the inner periphery of the tubular body and a number of exterior fins disposed on the outer periphery of the tubular body, thereby providing a large heat transfer surface area which in turn leads to a high heat exchange efficiency.
Typical examples of such known heat exchangers are disclosed in Japanese Patent Laid-open Publication Nos. 59-134496 and 59-173654. The heat exchanger disclosed in the first-mentioned Japanese publication includes a tubular body having a plurality of interior fins extending longitudinally thereof and projecting radially inwardly from the inner peripheral surface of the tubular body, and a plurality of exterior fins force-fitted over the outer periphery of the tubular body at angular intervals. The heat exchanger shown in the last-mentioned Japanese publication comprises a tubular body with a plurality of interior and exterior fins extending longitudinally thereof, the exterior fins being formed by cutting the material of the body itself.
Due to an additional working process required for forming the exterior fins, the disclosed heat exchangers are costly to manufacture and cannot be produced at a high rate. Furthermore, with the non-integral, force-fitted exterior fins, the two-piece heat exchanger such as shown in the first-mentioned Japanese publication has only a limited heat transfer efficiency due to a high thermal resistance at the joint between the body and the exterior fins. To improve the heat exchange efficiency, a large heat exchanger body is needed and thus a compact and light combustion heater is difficult to achieve.